Transparent polycrystalline ceramics are known in the art. There are, however, two significant difficulties in the preparation of such ceramics: 1) randomly oriented polycrystalline anisotropic ceramics, which inherently have a different refractive index along at least two crystal axes, scatter light at each grain boundary and 2) ceramics comprising two or more phases having different refractive indices scatter light at each phase boundary. Second phases include pores which may be present in ceramics. Such pores contain gases which have a refractive index of about 1.0 whereas the refractive index of a ceramic is typically significantly greater than 1.0 (e.g., in the range of 1.4 to 2.8).
Polycrystalline aluminum oxide is used as an optically transparent ceramic in certain applications (e.g., high pressure sodium vapor discharge lamps). The transparency of polycrystalline alumina, however, is limited because of its anisotropic crystal structure. An alternative to alumina is gamma-aluminum oxynitride. Gamma-aluminum oxynitride, commonly referred to as "AlON", is an AlN-Al.sub.2 O.sub.3 solid solution. In the early literature this material was sometimes referred to as "nitrogen stabilized cubic Al.sub.2 O.sub.3." Gamma-aluminum oxynitride is more transparent than alumina because the former has a cubic crystal structure which is inherently isotropic Whereas the latter has a non-cubic crystal which is inherently non-isotropic.
Synthesis of aluminum oxynitride was first reported by Yamaguchi et al. in "Study on the Reductive Spinel--A New Spinel Formula AlN-Al.sub.2 O.sub.3. Instead of the Previous One Al.sub.3 O.sub.4 ", Bull. Chem. Soc. Jap., 32, (11), Nov., 1959, pp. 1264-65, wherein alumina and graphite were reacted above 1650.degree. C. in an unspecified atmosphere. The composition and structure of gamma-aluminum oxynitride were later described in more detail by Lejus in "On the Formation of High Temperature Nonstoichiometic Spinels and Derivative Phases, In Several Systems Based on Alumina and In The System Aluminum Nitride-Alumina", Rev. Hates Temper. et Refract., Ch. 5, 1, 1964, pp. 58-95. Lejus's preparation of aluminum oxynitride included reacting aluminum nitride and alumina.
U.S. Pat. No. 4,241,000 discloses a structural ceramic material comprising sintered single phase, polycrystalline, cubic aluminum oxynitride which displays isotropic optical, thermal, and electrical properties, an infrared cutoff at about 5.2 micrometers, and which shows no chemical or physical property change after heating to 1100.degree. C. in an air atmosphere. The aluminum oxynitride ceramic was prepared by isostatically pressing a mixture of aluminum nitride and alumina powders, heating in nitrogen for 24 hours at 1200.degree. C., and then sintering in nitrogen at 1975.degree. C. for 1 hour.
Use of sintering aids, such as boron, yttrium, lanthanum compounds, or combinations thereof, to improve the optical transparency of sintered Alon is disclosed in U.S. Pat. Nos. 4,481,300, 4,520,116, 4,686,070, and 4,720,362 and in unexamined Japanese Patent No. SHO60-191061 published Sept. 28, 1985. The latter also describes an improvement in optical transparency by using aluminum nitride powder with a mean particle size diameter of less than 2 micrometers.
Weiss et al. in "The System Al--Mg--O--N", J. Am. Ceram. Soc., 65, (5-6), 1982, pp. C-68-69, reported that although an understanding of the phase relations and compositions of the Al--Mg--O--N system is still incomplete, gamma aluminum magnesium oxynitride may be described by the formula, EQU Mg.sub.y Al.sub.3-y-1/3X [].sub.2/3x O.sub.3+x+y N.sub.1-x-y,
wherein the temperature-dependent solubility limits are O.ltoreq.x.ltoreq.1; O.ltoreq.y.ltoreq.1; and x+y=1. The authors did not disclose or suggest a transparent gamma-aluminum magnesium oxynitride, nor did they disclose a possible use of this ceramic material.
U S. Pat. No. 3,026,210 discloses the use of up to 0.5 weight percent MgO as a sintering aid in the preparation of transparent Al.sub.2 O.sub.3. The presence of higher amounts of MgO resulted in a reduction in transparency due to the increased formation of a MgAl.sub.2 O.sub.4 second phase.
The strength of a ceramic is inversely related to its grain size. The average grain size of the aforementioned transparent alumina and aluminum oxynitride-based ceramics is typically in the range of 25 to 200 micrometers. The relatively large grain size results from the long sintering times (i.e., 20 to 100 hours) and high sintering temperatures (i.e., 1600.degree. to 2000.degree. C.) needed to remove residual pores.
One way of limiting grain growth is to utilize a composite of two materials which are in equilibrium with each other at the sintering temperature. The presence of two different crystal types limits the volume in which the individual crystals (grains) can grow without impinging on each other. The difficulty with this approach of preparing a fine-grained ceramic is that rarely do two material have refractive indices similar enough to avoid scattering.
Alumina and aluminum oxynitride have refractive indices of 1.76 and 1.77, respectively. Even with this degree of index matching, significant scattering may occur which severely limits the transparency of the composite.
Although European Patent Application No. 0107571 discloses a composite comprising alumina and aluminum oxynitride, there was no suggestion that it was transparent.